Vacuum-tube-filament construction and method of manufacturing same



Patented Dec. 14, 1926.

UNITED STATES IIEEEDIERIIIGJK S. MoCUIi-LQUGH, OF WITJKINSBURG, PENNSYLVANIA.

' VAGUUM-TUBE-FILAMENT CONSTRUCTION AND METHOD OF MANUFACTURING SAME.

No Drawing.

1 My invention relates to evacuated electrical devices and more especially to the cathode structure employed in such devices and to methods of manufacturing the same.

a One object of my invention is to provide a cathode construction which is particularly applicable to power-tube devices and which is of unusual rigidity and strength of construction and lends itself to inexpensive 1o quantity-production methods of manufacture.

Another object of my invention is to provide a coated cathode element, of the class described, which is characterized by an extremely high electron emissivity and by the absence of deterioration or disintegration, particularly of the coating, during normal operation. Q

A further object of my invention is to pro- 2 vide a cathode which is coated with thorium oxide and which has such high electron emissivity at temperatures below the volatilization point of the thorium oxide as to give the required saturation current.

In prior-art cathode constructions, it has been customary to coat a base metal with oxides of barium and strontium. Such coatings were deposited upon the base metal in the form of a closely adhering film. n the practical operation of such cathodes, the

heavy electrostatic forces established thereupon caused a disintegration of the oxide coatings thereof.

- According to my invention, I provide a cathode construction, for obviating the above-mentioned difficulties, comprising a base-metal core having an oxide coating on the outer surface thereof, said coating being partially fused into the base metal to pro- 4t vide, on the surface of the base metal, a crater-like film having an oxide surface.

Other objects and applications of my invention, aswell as details of construction and arrangement and operation, will appear more fully hereinafter, in connection with the accompanying description and claims.

lln order to complete the disclosure of my invention, I shall now describe one method of making a cathode having a crater-like W metallic film on the surface of a base metal, the outer surface of said'metallic film being oxidized. r

The base metal, which is to be treated as hereinafter described, may be in the form w of a rod comprising tungsten metal powder and a relatively small percentage of carbon.

ing of desired thickness on the surface of the Application filed October 24., 1922; Serial No. 556,615.

The outer surface of the rod is immersed in a coating solution of paste comprising a mixture of some metallic powder, as tungsten, and an ammonium solution ofthorium oxide. While I have specified ammonia as the dissolving agent, I do not desire to be limited thereto. The immersion may be repeated a number of times to form an aggregate coatrod.

When the coating process is completed, the coated rod is baked in a furnace having 9. temperature of approximately 400 C. for such time as may be necessary to volatilize the ammonia from the coating. The base metal is then sintered, as by passing a heavy current therethrough in an atmosphere of hydrogen. During the sintering process, the portion of the thorium oxide coating in contact with'the base metal is reduced to the metallic state and is fused or alloyed into the surface'of the base metal, forming eraters or indentationstherein.

The layer of material between the tungsten of the rod on the one hand, and the thorium oxide coating on the other, may be designated as a solid solution.

I desire to point out, however, that, during this operation, the outer surface of the thorium coating is only partially reduced to the metallic state. Thus, I have formed a cathode wherein the surface is provided with indentations or craters, and the outer surface of which is in the form of a complete or partial oxide covering. Although I do not wish to be restricted to any particular theory, it is my opinion that the craters have a useful function in increasing the electronemissive surface. It is thought that the reduction of the thorium oxide to a metallic state and the resulting fusion with the base metal is greatly facilitated by the carbon which was included in the material of the original rod. It is further thought that the 0 formation of the craters is greatly aided by the presence of hydrogen in the material of the base metal, as well as in the atmosphere surrounding; the same. It is understood, '9 however, that my invention, in its broader aspects, is not to be specifically limited to such theories of operation.

-Following the sintering process, just described, the rod is passed through a hydrogen furnace and worked into substantially mm the desired shape.

While I have described only one method of demanded by the prior art.

I claim as my invention:

1. The method of manufacturing filaments which consists in depositing upon a metal base a coating, reducing a portion of said coating to a metallic state and fusing said portion to said base, whereby craters are formed extending into the surface of said base.

2. The process of forming a pitted surface which consists in depositing an oxide o0ating upon a metal base and sintering said base and a portion of said coating to form craters on the surface extending into the base metal, said craters having an oxide surface.

3. The process of manufacturing coated filaments which consists in immersing a metal base in an ammonium solution of thorium oxide to form a coating for the base, heating said coating to remove the ammonia,

reducing substantially that portion of the coating in contact with the base to a metallic state, and forming a junction with said base by fusion 4. The process of manufacturing filaments which consists in forming a body containing tungsten and carbon, immersing said body in a solution of thorium oxide to form a coating for said body, removing the means dissolving said vthorium oxide, and sintering said body and portions of said coating in immediate contact therewith whereby craters are formed in the surface of said body, said craters having an oxide coating.

5. The step in the manufacture of filamentary structures which consists in fusing a thermionically active oxide coating on a metallic core in the presence of carbon and hydrogen, in such manner as to form craterlike depressions.

6. The step in the manufacture of filamentary structures which consists in fusing a thoria coating on a tungsten core in the presence of carbon and hydrogen, in such manner as to form crater-like depressionsj In testimony whereof, I have hereunto subscribed my name this 13th day of October 1922. w l

FREDERICK s. MGOULLOUGH. 

